The invention relates to a method of producing hot strip from unalloyed or low-alloy steels with carbon contents in the range of 0.3-0.9% by the following steps:
austenitization of a slab, PA1 hot rolling of the heated slab, PA1 cooling of the strip and PA1 coiling of the strip.
Hot strip from such steels is used for direct further processing by working or for the production of cold rolled strip. The finished parts made of these steels are normally subjected to a heat treatment by hardening and annealing to adjust the required strength and hardness values.
Due to the carbon contents, hot strip made from these steels has high tensile strength. It depends on the proportion of pearlite in the structure and on the formation of pearlite. In the case of steels having carbon contents between 0.4 and 0.7%, an increase in the proportion of pearlite in the structure from 50-100% produces an increase in tensile strength from 600 to 1100 N/mm.sup.2 (Journal of the Iron and Steel Institute, 205, 1967, Page 653/664). The proportion of pearlite in the structure can be increased while at the same time the quantity of ferrite is reduced, if the cooling rate of the strip is high in the zone of the austenite/ferrite transformation. Furthermore, the cooling rate in the zone of the austenite/ferrite transformation affects the lamellar distance of pearlite and therefore also affects strength. In the case of a steel with 0.72% C and 0.73% Mn, an increase in the cooling rate from 5 to 30 K./sec reduces the lamellar distance of pearlite and thereby increases the tensile strength from 950 to 1300 N/mm.sup.2 ("Atlas of the Heat Treatment of Steels", published by Stahl-Eisen, Duesseldorf, 1961, Table II-101 E and Mem. Sci. Revue de Metallurgie 75, 1978, pages 149/159).
In practice, when hot strip is produced from steels having relatively high carbon contents, the hot strip is heavily cooled by water on the run-out roller table of the hot strip mill. The object of this method is to equalize mechanical properties and structure over the length of the hot strip ("Stahl and Eisen", 89, 1969, pages 815/824). As already described, the high cooling rate due to the heavy water cooling increases the proportion of pearlite and reduces that of ferrite, while reducing the lamellar distance of the pearlite. As already explained, both changes cause an increase in the strength of the hot strip.
With this method of production, the following typical properties of hot strip are obtained for two steels according to DIN 17200 and DIN 17220:
______________________________________ Steel Tensile Strength Hardness ______________________________________ C 45 .gtoreq.800 N/mm.sup.2 .gtoreq.95 HRB C 75 .gtoreq.1000 N/mm.sup.2 .gtoreq.25 HRC ______________________________________
The mean lamellar distance of pearlite is between 0.1 and 0.2 .mu.m.
For the direct further working of hot strip thus produced, by bending, adjusting, coiling or punching, or for the production of cold rolled strip, the high strengths result in high forming forces and throw a heavy load on installations. The result is both high energy costs and a shortened service life of installations.
Methods are known (European Patent Specifications 0 019 193 and 0 099 520) in which the incorporation of an additional device on the run-out roller table of the hot strip mill partially reduces the cooling rate in the austenite/ferrite transformation, namely by coiling of the hot rolled strip in an incubator. In both, the prior art processes the time of exposure of the strip in the incubator is less than 2 minutes. Then the strip is uncoiled and cooled and coiled in a conventional coiling installation. The object of reducing cooling rate by coiling in the incubator is to encourage ferrite formation, thereby increasing the proportion of ferrite in the structure. However, the steels mentioned in these two Specifications have low carbon contents, and their structures consist mainly of ferrite. In contrast, the main object of the method according to the invention is to reduce tensile strength by increasing the lamellar distance of pearlite--i.e., that structural component representing more than half the formation of the structure in the case of the pearlitic-ferritic steels in question.
It is an object of the invention to reduce the tensile strength of hot strip of unalloyed or low-alloy steel with 0.3-0.9% C, without affecting the uniformity of its properties or the structural formation over the length and width of the hot strip.